A potential problem with moving a part DOWN to a sub-assembly with restructure is that you MAY have to have the upper level assembly in memory to open the sub-assembly. Because I don't know how references to skeletons behave in this instance. Certainly, if you have other types of references (such as placement datums) to the upper level assembly, you will not be able to open the sub-assembly by itself. Moving parts UP to a higher level assembly never causes a problem.